1. Field of the Invention
The present invention generally relates to a black-white cholesteric liquid crystal display and a method for manufacturing the same and, more particularly, to a black-white liquid crystal display using a cholesteric liquid crystal material and a method for manufacturing such a liquid crystal display using polymerization/phase-separation, resulting in diffused polymeric molecules and polymeric cell walls, so as to minimize the manufacturing time, strengthen the polymeric cell walls and enhance the contrast ratio as well as the brightness.
2. Description of the Prior Art
In recent years, the development in flat panel displays such as “electronic papers” and “electronic books” has been tremendously growing. For use in the flat panel displays, there are several types of display cells including the liquid crystal display, the electrophoresis display, the electrochromism display, the electrolysis display, etc. Especially for use in electronic papers, the cholesteric liquid crystal display has advantages over others in brightness, contrast ratio and is easier to drive.
In the U.S. Pat. No. 5,847,798, entitled “Polymer Stabilized Black-White Cholesteric Reflective Display”, Yang and Ma (Kent State University) disclose a cholesteric liquid crystal display. As shown in FIG. 1A and FIG. 1B, which are schematic diagrams showing respectively the planar state and the focal conic state of a conventional cholesteric liquid crystal display. The cholesteric liquid crystal display exhibits two stable states at zero electro field applied, namely, the planar state and the focal conic state. In FIG. 1A and FIG. 1B, the cholesteric liquid crystal display 10 comprises a substrate 11, on which is covered a transparent electrode 12 such as an indium-tin oxide (ITO) film or the like. The cholesteric liquid crystal display 10 is filled with a cholesteric liquid crystal material 13, which is mixed with dispersed phase-separated polymer domains 14. On one side of the substrate 11 is stacked an absorption layer 15, usually implemented as black. Furthermore, an AC voltage source 16 is connected to the electrodes 12 in order to switch the liquid crystal display 10 between different optical states.
In FIG. 1A, a predetermined voltage value 16 is applied to the cholesteric liquid crystal display 10 and then removed to place the liquid crystal material 13 in the planar state. When a cholesteric liquid crystal material 13 is in the planar state, the helical axes are more or less perpendicular to the surface of the crystal display 10. When an enviroment light 17, as seen by an eye, is incident on the cholesteric liquid crystal display 10, as represented by an arrow with the numeral designation 17, light is reflected as represented by an arrow with the numeral designation 18. The central or primary wave length of the reflection band depends on the pitch, average refractive index and birefringence of the cholesteric liquid crystal material.
In FIG. 1B, a predetermined voltage value 16 is applied to the cholesteric liquid crystal display 10 and then removed to place the liquid crystal material 13 in the focal conic state. When a cholesteric liquid crystal material 13 is in the focal conic state, the helical axes are more or less randomly arranged. When an enviroment light 17, as seen by an eye, is incident on the cholesteric liquid crystal display 10, as represented by an arrow with the numeral designation 17, little of reflected light as represented by an arrow with the numeral designation 18 goes through the upper substrate 11. Moreover, an absorption layer 15 with a black color can be used such that the polymer domains associated with the focal conic state appear black.
However, the above-mentioned invention has a problem in that it takes about 1.5 hours to form the dispersed polymer domains because the dispersed polymer domains are formed under relatively weaker UV light without masks.
Moreover, for the application of flexible displays, polymeric walls are popularly used to strengthen the cell gap. The polymeric walls are formed using polymerization/phase-separation as disclosed in the U.S. Pat. No. 5,473,450 filed by Sharp Kabushiki Kaisha (Japan). In order to further strengthen the cell gap, micro-capsules are formed by micro-encapsulating polymer molecules and liquid crystal molecules in the display cell, as described in the U.S. Pat. No. 6,120,701 and No. 6,203,723 filed by Hsu.
Therefore, there is need in providing a black-white liquid crystal display using a cholesteric liquid crystal material and a method for manufacturing such a liquid crystal display using polymerization/phase-separation, resulting in diffused polymeric molecules and polymeric cell walls, so as to minimize the manufacturing time, strengthen the polymeric cell walls and enhance the contrast ratio as well as the brightness.